1. Field of the Invention
The present invention relates to a switching power supply that prevents AM radio broadcast from receiving disturbance due to power supply noises, and reduces the power supply noises.
2. Description of the Related Art
A switching power supply inevitably generates high frequency power supply noises, electromagnetic interference (EMI) noises, due to switching operation of a semiconductor switching element as shown in FIG. 4. The power supply noise causes AM radio broadcast disturbance when the frequency of the power supply noise overlaps the AM radio frequency band. In order to reduce the power supply noise, i.e. an electromagnetic radiation noise due to the switching operation in a switching power supply, Patent Document 1 (identified further on) discloses modifying the power supply frequency, i.e. switching frequency, with random fluctuation i.e. jitter.
Patent Document 2 (identified further on) discloses a method to avoid the radio receiving disturbance, in which the power supply frequency, i.e., the switching frequency of a switching power supply is detected from the output voltage of the power supply, and if the difference is small between the higher harmonic of the detected switching frequency and the receiving frequency by the AM radio receiver, the switching frequency of the power supply is changed. A frequency band of AM radio broadcast includes, as shown in FIG. 5, a bandwidth BW of upper and lower side bands around the central carrier frequency fc, which is a so-called receiving frequency. Thus, radio receiving disturbance can be avoided if the difference mentioned above is greater than the side band width BW/2.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2003-153526 [Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2005-130081
However, even if the power supply noise is reduced by using a jitter control technique to the switching frequency as disclosed in Patent Document 1, the disturbance to receiving AM radio broadcast is not necessarily avoided with the reduced power supply noise. The jitter control of the switching frequency causes the power spectrum of the power supply noise to not be concentrated at a specific frequency but diffused over a frequency axis. As a result, it becomes more likely for the frequency of the power supply noise to overlap or interfere with the frequency band of the AM radio broadcast, thus causing disturbance to receiving the AM radio broadcast.
The method disclosed in Patent Document 2 compares a predetermined reference value with the difference between the receiving frequency fc of an AM radio broadcast and the higher harmonic frequency of the order n (n is a natural number) of the switching frequency of the switching power supply. According to the comparison result, the oscillation frequency, i.e. the switching frequency, of the switching power supply is simply shifted stepwise.
Consequently, if the oscillation frequency, i.e. switching frequency, of the switching power supply contains itself an error due to scattering of characteristics of circuit components of the switching power supply, the shift control of the oscillation frequency of the switching power supply according to solely the difference would divert the oscillation frequency, i.e., the switching frequency or the power supply frequency, of the switching power supply significantly from the frequency of the specification of the power supply. This may create a problem that the filter provided in the switching power supply cannot remove enough conduction noise emitted from the switching power supply through a power supply line.